These studies are targeted towards the development of novel diagnostic modalities and specific immunotherapies for psoriatic arthritis, a relatively common chronic inflammatory disease affecting several organs including skin and joints. Patients with known or suspected psoriatic arthritis are evaluated at the Clinical Center. Studies include characterization of the clinical and laboratory features of the disease, research studies of the natural history and pathogenesis, genetic studies, and determination of the patients eligibility to enter experimental therapeutic protocols. Patients are asked to contribute blood, skin or synovial membrane samples for immunologic and genetic research studies. Immunologic studies include investigations of 1) the cytokine profile of the disease (type 1 vs type 2 immune response); 2) the regulation of apoptosis of keratinocytes and synoviocytes; 3) the role of cytotoxic (CD8+) T cells in the pathogenesis of the disease; and 4) the T cell antigen receptor repertoire in the peripheral blood and at sites of inflammation (i.e., skin and joints).